Improperly inflated tires have been recognized as the source of a significant number of automobile accidents. In addition, under-inflated tires have shorter lifetimes and decrease vehicle fuel efficiency. This problem has increased because of the almost universal acceptance of self-service stations for refueling vehicles and the longer periods of time between preventive maintenance service provided by newer vehicle designs and synthetic motor oils that have extended the period between oil changes. As a result, the responsibility for checking the pressure in the vehicle's tires has fallen on the driver. Unfortunately, a significant fraction of the vehicle owners and drivers fail to check the tire pressure in their vehicles until the pressure has become so low that the tire looks “flat”. However, the tire can become unsafe long before it appears flat.
Even a properly inflated tire loses air at a significant rate. It is estimated that a damage-free properly inflated tire can lose air at the rate of 1 psi per month. Once the tire becomes under inflated, the rate of loss can become greater. New cars with service intervals of 5,000 to 10,000 miles are currently on the market. Hence, the period between preventive maintenance checks is sufficient to allow even a damage-free tire to lose sufficient air to become a safety hazard if the owner does not periodically check the tire and re-inflate it.
In an effort to address this serious safety problem, legislation requiring tire pressure monitoring equipment has been passed. This legislation requires the subject vehicles to have a tire pressure warning system that continuously monitors the tire pressure and warns the driver if the tire is significantly under inflated. In principle, such a low-pressure warning will cause the driver to seek out an air station and properly inflate his or her tire.
Consider a driver who starts his or her car in the morning in her garage and receives a low-pressure warning from one of these mandated systems. Assume that the under-inflated tire had only 25 psi, and the manufacturer recommends 30 psi at ambient temperatures. The typical vehicle owner does not have an air compressor in his or her garage. Hence, the driver must drive to a service station on the way to work. After the driver has driven a few miles, particularly at high speeds, the temperature of the tires on the vehicles will have substantially increased. Hence, when she arrives at the gas station, the pressure in her tires will have increased to say 28 psi. If the driver then proceeds to fill the tire to the manufacturer's suggested rating of 30 psi, the tire will still be under-inflated by about 3 psi. That is, when the tire cools to ambient once again, the tire pressure will only be about 27 psi.
This problem arises because the driver does not know the correct pressure reading to use with the gas station air hose when she inflates her heated tire. It should be noted that a difference in inflation pressure of only 4 psi is claimed to have been the difference between a safe and unsafe tire condition in the Ford Explorer tire failures blamed for a significant fraction of rollover accidents associated with these vehicles.
The above description also assumes that the driver knows the correct pressure for each of the tires when the tires are cold. The currently mandated pressure warning system merely informs the driver that the tire pressures is significantly below the desired level by lighting an appropriate light on the car's instrument console. It is up to the driver to know correct pressure with which to inflate each of the tires. The warning light only provides an indication of a dangerously low tire pressure, not the optimum tire pressure. In this regard, it should be noted that the optimum pressure is typically different for the front and rear tires. The correct values are listed in the vehicles owner's manual or on decals on the body of the vehicle. However, this manual or the decals are often missing, particularly in older vehicles. In addition, the correct inflation pressure is tire dependent; thus if the original equipment tires are replaced by different tires, the values can be in error.
Finally, the above scenario assumes that the driver is competent to use the service station air station to fill the tire to the correct level and that the service station air station has a gauge that is accurate. The air hoses in most service stations have an air gauge that consists of a piston that is forced against a spring such that a tab having pressure markings extends from a housing. The user reads the highest pressure marking that is visible on the gauge when the air hose is pushed against the valve stem of the tire. The user fills the tire by depressing a trigger that actuates a valve that connects the hose to the pressure reservoir. During the filling process, the gauge does not provide an accurate reading of the pressure in the tire; hence the filling process is an iterative process in which the user adds air, measures the pressure, adds air, etc until the correct pressure appears on the gauge or the tire is over-inflated. If the tire is over-inflated, the user then uses the end of the hose to bleed air from the tire until the correct pressure reading is seen on the gauge.
The gauges are mechanical devices that have a significant amount of backlash. In addition, any dirt or debris that accumulates on the piston increases this backlash, and hence, makes an accurate reading difficult. Hence, significant errors can still be present even when the driver takes the time to fill the tires to the manufacturer's specification.